The regulating member of a mechanical watch is formed by a harmonic oscillator, the sprung-balance, whose natural oscillation frequency mainly depends on the inertia of the balance wheel and on the elastic rigidity of the balance spring.
The oscillations of the sprung-balance, otherwise damped, are maintained by the impulses provided by an escapement generally formed by one or two pivoting components. In the case of the Swiss lever escapement, these pivoting components are the pallet lever and the escape wheel. The rate of the watch is determined by the frequency of the sprung-balance and by the disturbance caused by the impulse from the escapement, which generally slows down the natural oscillation of the sprung-balance and thus causes a losing rate.
The rate of the watch is thus disturbed by any phenomena that can impair the natural frequency of the sprung-balance and/or the time dependence of the impulse supplied by the escapement.
In particular, following temporary exposure of a mechanical watch to a magnetic field, rate defects (related to residual field effects) are generally observed. The origin of these defects is the permanent magnetization of the fixed ferromagnetic components of the movement or of the external watch parts and the permanent or temporary magnetization of the moving magnetic components forming part of the regulating member (sprung-balance) and/or of the escapement.
After exposure to the field, the magnetically charged or magnetically permeable moving components (balance wheel, balance spring, escapement) are subjected to a magnetostatic torque and/or to magnetostatic forces. In principle, these interactions modify the apparent rigidity of the sprung-balance, the dynamics of the moving escapement components and friction. These modifications produce a rate defect which may vary from several tens to several hundreds of seconds per day.
The interaction of the timepiece movement with the external field, during exposure, may also result in stopping the movement. In principle, there is no correlation between stopping under a field and the residual rate defect, because stopping under a field depends on the temporary, sub-field magnetization of the components (and thus on the permeability and saturation field of the components), whereas the residual rate defect depends on residual magnetization (and thus, mainly, on the coercive field of the components) which may be low even in the presence of high magnetic permeability.
Since the introduction of balance springs made of very weakly paramagnetic materials (for example silicon), the balance spring is no longer responsible for rate defects in watches. Any magnetic disturbances still observable for magnetization fields lower than 1.5 Tesla are thus due to the magnetization of the balance staff and to the magnetization of the movable escapement components.
The pallet lever body and the escape wheel can be manufactured in very weakly paramagnetic materials without this affecting their mechanical performance. Conversely, the arbors of the movable components require very good mechanical performance (good tribology, low fatigue) to permit optimum, constant pivoting over time, and it is thus preferable to manufacture them in hardened steel (typically 20AP carbon steel or similar). Such steels are materials that are sensitive to magnetic fields because they have a high saturation field combined with a high coercive field. The balance staff and arbors of the pallet lever and escape wheel are currently the most critical components as regards magnetic disturbances of the watch.
In particular, the balance staff is the most sensitive component with respect to chronometry (residual effects), because a disturbing torque of magnetic origin acting on the arbor directly modifies the oscillation frequency of the sprung-balance, and this modification is, in principle, unlimited (it depends only on the intensity of the residual magnetic fields and on the rigidity of the balance spring), whereas a disturbance of the escapement function produces a rate defect that is limited to the nominal loss at the escapement (the resulting disturbance cannot be greater than the disturbance already produced by the escapement in normal conditions).
FR Patent Application 2275815A1 in the name of NIVAROX discloses the manufacture of a balance staff from a profile bar including several wings distributed around the pivot axis, and a variant with curvilinear wings.
FR Patent Application No 2090784A5 in the name of FEINMETALL discloses the assembly of a balance spring to a balance including a cross-member with two substantially symmetrical wings.
JP Patent Application No S6263884A in the name of ZENKOSHA TOKEI discloses the machine cutting of a balance comprising two wings.
WO Patent Application No 01/77759A1 in the name of DETRA discloses an escapement device comprising a train for transmission of energy to an oscillator capable of receiving this energy and of transmitting an oscillation frequency, and first means capable of producing at least a first portion of the energy which is transmitted by the train and intended to power the oscillator, wherein the first means are configured to provide mechanical torque that varies essentially as a function of the angle of rotational travel of the train, this mechanical torque having at least one stable position, and at least one unstable position, over a period of rotational travel of the train. In a particular embodiment, these first means produce a magnetic torque that varies as a function of time, by combining a diametrically magnetized rotor with a stator comprising cells in the bore receiving the rotor.